The present invention relates to a power unit for neon sign and particularly to a high-frequency power unit which boosts high-frequency power by a transformer to light neon or argon tubes connected to the secondary side thereof.
Conventionally, the commercial line power is boosted prior to its application to neon or argon tubes to light them, but this method necessitates the use of a large or bulky boosting transformer. In view of this, it has been proposed to utilize high-frequency power of, say, 20 or 30 kHz for lighting neon or argon tubes (hereinafter referred to simply as neon tubes) so as to permit the use of a small boosting transformer.
The power unit of this kind, which utilizes such high-frequency power, is usually capable of lighting neon tubes even if the number of tubes connected to the boosting transformer is in excess of a predetermined value. When a user inadvertently or recklessly connects neon tubes more than specified, the boosting transformer is subjected to abuse or forced to operate under severe conditions, causing sudden current and voltage increases and often resulting in a runaway of the transformer.
In the case where a plurality of neon tubes are connected in series to the transformer, if any one of the tubes is broken or falls off, the secondary side of the transformer becomes open. When all the neon tubes are being lighted, a resistance load is imposed on the transformer, but when the secondary side is open, the stray capacitance between the transformer and the ground is applied as a load to the former because high-frequency power is fed thereto. In this situation, a leading current flows in the boosting transformer and its voltage increases; for instance, voltage at the output side of the secondary winding becomes about twice higher than the voltage level during normal operation. A higher voltage appears particularly when the transformer is in the resonance state. This may sometimes give rise to the destruction of insulation of the transformer or damage to an inverter for obtaining the high-frequency power.
In FIG. 1 there are shown a conventional high-frequency power unit for neon tubes and a plurality of neon tubes connected in series between its output terminals 18 and 19. Commercial AC power from a commercial AC power source 5 is converted by a rectifier 10 in a high-frequency power circuit 100 to DC power, which is fed to an inverter 20. The inverter 20 converts the DC power to high-frequency power, which is applied across terminals 2 and 3 of a primary winding Wp of a neon transformer 17. A plurality of neon tubes 4 are connected in series between the output terminals 18 and 19 of a secondary winding Ws of the transformer 17. An equivalent circuit of this circuit connection is such as depicted in FIG. 2A, in which an inductance L.sub.T and an internal resistance r of the neon transformer 17 are connected in series between the terminals 2 and 18, the terminals 3 and 19 being directly connected to each other. The inductance L.sub.T is the sum of a leakage inductance M between the primary and secondary windings Wp and Ws and a self-inductance Ls of the secondary winding Ws. Between the terminals 18 and 19 there is connected a parallel circuit composed of a leakage inductance Lx, a winding stray capacitance C.sub.NS of the transformer 17 and a leakage resistance Rx. Since the neon tubes 4 are regarded as resistors during discharge, a resistor R.sub.L is connected as a load R.sub.L between the terminals 18 and 19, a capacitance C.sub.LL is connected between the neon tubes 4 in parallel to the load R.sub.L, and the capacitance C.sub.LG between each of the neon tubes 4 and the ground is present between each of the terminals 18 and 19 and the ground. Moreover, the capacitance C.sub.T between the core of the transformer 17 and each winding is expressed as the capacitance C.sub.T ' between the high-frequency voltage source 100' and the ground. Letting the high-frequency output voltage of the high-frequency power circuit 100 in FIG. 1 be represented by V.sub.AC and the numbers of turns of the primary and secondary windings Wp and Ws by n.sub.p and n.sub.s, respectively, a voltage Vs=V.sub.AC n.sub.s /n.sub.p is applied across the terminals 18 and 19. Therefore, in FIG. 2A the equivalent high-frequency voltage source for the boosted high-frequency voltage Vs is identified by 100'.
Adding up the respective capacitance components, the equivalent circuit of FIG. 2A can be represented as shown in FIG. 2B. The series connection of the resistance r and the inductance L.sub.T is connected at one end to the terminal 2, and the capacitance C.sub.LL and the resistance R.sub.L are connected in parallel between the other end of the above-mentioned series connection and the terminal 3. The leakage inductance Lx and the leakage resistance Rx are usually large and currents therein are negligibly small. While the neon tubes 4 are being normally lighted, the resistance R.sub.L is very small; hence, as shown in FIG. 2C, a current I flowing across the inductance L.sub.T is substantially in-phase with a voltage Vo that is developed across the load, a voltage V.sub.LT that is developed across the inductance L.sub.T leads the current I by a phase angle of around 90.degree., and a voltage Vr that is developed across the resistor r is substantially in-phase with the current I. The vector sum of the voltages Vo, V.sub.LT and Vr is the voltage Vs; therefore, Vs&gt;Vo.
In the event that one of the neon tubes 4 is broken or cracked, that is, when the terminals 18 and 19 are disconnected from each other, a capacitance C', which is the sum of the capacitance C.sub.LL between the neon tubes 4 and the capacitance C.sub.LG between each neon tube and the ground, is imposed as a load on the transformer. Since the capacitance C' is relatively large when the number of neon tubes 4 is large, and since the high-frequency voltage Vs is applied, the impedance of the capacitance C' is relatively small. Hence, as shown in FIG. 2D, the current I flowing through the inductance L.sub.T leads the load voltage V.sub.0 by a phase angle of about 90.degree., the voltage V.sub.LT which is developed across the inductance L.sub.T leads the current I by a phase angle of about 90.degree., and the voltage which is developed across the resistor r is in phase with the current. The vector sum of the voltage Vr, V.sub.LT and V.sub.O is the applied voltage Vs, and the voltage Vr becomes abnormally higher than the voltage Vs, the transformer entering the overvoltage stage.
To prevent this, it is a general practice in the prior art to cut off the current in the primary side of the neon transformer upon detection of flowing of an overcurrent in the primary winding by a detector, or upon detection of a discharge or spark that is generated across a discharge or spark gap formed in a part of the secondary winding when an overvoltage is developed thereacross.
In the conventional neon tube lighting high-frequency power unit, the current in the primary side is cut off after detection of the overcurrent or overvoltage state of the transformer as mentioned above; however, this does not provide sufficient protection of the power unit because an electric breakdown of the secondary winding or breakdown of the inverter for applying the high-frequency power to the transformer is already caused by the overcurrent or overvoltage when the current cutoff takes place.
In a display of the type employing high-frequency driven neon tubes, though dependent on their diameters or gas pressures, stripe patterns commonly referred to as "jelly beans" may sometimes appear on the neon tubes lengthwise thereof during their ON state. With the prior art, it is impossible to prevent the jelly beans from occurrence.
In FIG. 3 there is shown a prior art example of a small capacity half-bridge inverter, indicated generally by 20, that is used in the neon tube lighting high-frequency power unit 100 of FIG. 1. A full-wave rectifier 15 is connected via a switch 14 across the AC power input terminals 11 and 12. A series circuit of capacitors C1 and C2 and a series circuit of switching elements SW1 and SW2, formed by FETs, are connected via a delay switch circuit DSW across the output of the rectifier 15. The primary winding Wp of the neon transformer 17 is connected between the connection point of the capacitors C1 and C2 and the connection point of the switching elements SW1 and SW2. Moreover, a smoothing circuit, which is formed by a parallel connection of a capacitor 16C and a resistor 16R, is connected between the output terminals of the fullwave rectifier 15.
The smoothing circuit 16 is connected at its positive side via a resistor 21 to a power terminal of a switching regulator 22 for generating a high-frequency switching signal, the negative side of the smoothing circuit 16 being connected to a grounding terminal of the switching regulator 22. The switching regulator 22 is a commercially available integrated circuit. A capacitor 23 and a Zener diode 24 are connected in parallel between the power terminal and the grounding terminal of the switching regulator 22. A switching control signal output of the switching regulator 22 is connected via a capacitor 25 to a primary side 26P of a pulse transformer 26. Secondary windings 26S1 and 26S2 of the pulse transformer 26 have their both ends connected to gates and source of the FETs that form the switching elements SW1 and SW2, respectively.
When AC power is supplied to the rectifier 15, the resulting direct current begins to charge the capacitor 23 via the resistor 21, and when the voltage of the capacitor 23 exceeds a certain value, the switching regulator 22 starts its oscillation. After this, the power terminal of the switching regulator 22 is held at a voltage that is determined by the Zener diode 24, relative to the ground. The switching regulator 22 generates a rectangular high-frequency signal and applies it via the capacitor 25 to the primary winding 26P of the pulse transformer 26. A time constant that is dependent on the values of a resistor 20R and a capacitor 20C in the delay switching circuit DSW is chosen such that the smoothing circuit 16 starts a DC supply and then a transistor switch TSW of the delay switching circuit DSW is turned ON at a timing ten-odd cycles after the start of oscillation of the switching regulator 22.
The input rectangular signal to the primary winding 26P of the pulse transformer 26 is applied intact to the gate of the switching element SW1 from the one secondary winding 26S1 and in the inverted polarity to the gate of the switching element SW2 from the other secondary winding 26S2. Accordingly, the switching element SW1 is turned ON at the rise of the output rectangular wave from the switching regulator 22, whereas the switching element SW2 is turned ON at the fall of the rectangular wave. By turning ON the switching elements SW1 and SW2 alternately with each other, the capacitors C1 and C2 alternately discharge through the neon transformer 17, outputting therefrom high-frequency power. Incidentally, the neon transformer 17 has a tertiary winding Wt. Upon initiation of supplying the high-frequency power to the primary winding Wp of the neon transformer 17 through the alternate ON-OFF operation of the switching elements SW1 and SW2, the AC output from the tertiary winding Wt is provided via a diode 29D to the switching regulator 22, thus starting power supply thereto from the transformer 17.
In this-prior art inverter 20, however, the pulse transformer 26 may sometimes gets saturated at the start of operation, with the result that the amplitude of its drive signal output is unstable for several cycles at the start of operation. To prevent this, the delay switching circuit DSW is provided so that no current is supplied to the switching elements SW1 and SW2 for a period of ten-odd cycles after the start of operation of the inverter 20, that is, no current supply to them takes place before the oscillation of the switching regulator 22 becomes stable. In this instance, however, the delay switching circuit is inevitably bulky and expensive because the transistor switch TSW needs to control a relatively large current.